1. Field of Invention
The present invention relates in general to impact sensors. More particularly, the present invention relates to an impact sensor that includes a piezoelectric transducer operatively connected to a device that changes color states in a bistable manner in response to electric power generated by the piezoelectric transducer when exposed to impact force.
2. Background Art
Impact indicators are commonly used to show when a product has been subjected to an impact exceeding a specified level. Impact indicators are typically used to monitor products that are fragile or otherwise sensitive to impact. Impact may occur, for example, during the product's manufacture, transportation or storage.
The impact indicator, which is typically a disposable, single-use device, is typically affixed directly to the product or packaging. The impact indicator detects and records an impact exceeding a user-specified detection threshold (e.g., 50 G for 50 ms).
Impact indicators are typically available with different sensitivities (e.g., 10 G for 50 ms, 25 G for 50 ms, 50 G for 50 ms, 75 G for 50 ms, and 100 G for 50 ms), each providing a different user-specified detection threshold. Such impact indicators are typically limited to a maximum user-specified detection threshold of about 100 G (50 ms).
Impact indicators typically employ the fracturing of a dye filled glass tube, rupturing of tiny dye filled spheres, or other rigid materials sensitive to impact. These devices generally provide a qualitative measure of severe impact and do not indicate how much force was applied to a monitored product or material.
For example, the impact indicator will activate only if the user-specified detection threshold is reached (it will not activate if the detection threshold is not reached). If the impact indicator activates, it does so in a manner that does not quantify the amount by which the user-specified detection threshold may have been exceeded. This information may be useful. On the other hand, if the impact indicator does not activate, it does not provide any indication of a near-miss with respect to the user-specified detection threshold. Again, this information may be useful.
Another solution is to use a conventional impact recorder that electronically measures information such as impact force and duration, as well as the time and date of impact. For example, the ShockLog RD298 impact and environmental recorder (available from ShockWatch, Inc Dallas, Tex.) uses triaxial piezoelectric accelerometer technology to record direction, amplitude, and duration of impact force, as well as the date and time of up to 400 shock and vibration events. Though more sophisticated and robust than impact indicators, such conventional impact recorders are relatively costly.
Therefore, a need exists for an enhanced impact sensor that will detect and measure rough handling and impact damage.